The present invention relates to a process for preparing high-purity 2,2-bis(4-hydroxyphenyl)propane (referred to as bisphenol A hereinafter).
Bisphenol A is used as a raw material for polycarbonate resins and epoxy resins and also for engineering plastics Colorless and high-purity bisphenol A is required for these uses.
Bisphenol A is prepared by the reaction of acetone with excess phenol in the presence of an acid catalyst such as hydrochloric acid. The product mixture contains bisphenol A and also the catalyst, unreacted acetone, unreacted phenol, water, and other by-products such as coloring substances.
There are many known processes for obtaining high-purity bisphenol A from the product mixture. For example, in the case where hydrochloric acid has been used as the catalyst in the reaction, the product mixture is heated at 110 to 120.degree. C. under reduced pressure, thereby removing hydrochloric acid, unreacted acetone, water, and a small amount of phenol and thereafter bisphenol A in the form of an adduct with phenol is separated by cooling. The other process includes distillation to separate bisphenol A from other substances having a higher and lower boiling point than that of bisphenol A. The thus-obtained bisphenol A may be further purified by extraction with a solvent or recrystallization from a solution.
In the case where hydrochloric acid has been used as the catalyst, the product mixture which has been distilled to remove hydrochloric acid, acetone, and water still contains a trace amount of hydrochloric acid which causes some trouble in the subsequent purification steps.
One trouble is the corrosion of equipment due to the acid. The corrosion yields metal salts which contaminate bisphenol A, and the removal of the metal salts requires a complicated purification procedure. A possible countermeasure is to use equipment made of an acid-resistant material; however, this is not economical because such equipment is expensive.
Another trouble is that bisphenol A is decomposed due to the acidic substance during distillation, as described in U.S. Pat. No. 3,073,868 and Japanese Patent Publication No. 4875/1963.
A crystallization process of the adduct of bisphenol A with phenol is disclosed in Japanese Patent Laid-open No. 135832/1983. In the process, water is added to the product mixture and the product mixture is cooled by evaporating a water-phenol mixture, thereby attaining crystallization In the evaporation step, a trace amount of hydrochloric acid distills together with water and phenol. If the distillate is discarded, hydrochloric acid does not accumulate, but discarding the distillate is economically undesirable because the phenol is lost. If the distillate is recycled and reused, hydrochloric acid accumulates and the accumulated hydrochloric acid causes some corrosion of equipment and some decomposition of bisphenol A in the subsequent steps.